Download Dec #2 - nydxa

P20 December 1997
with an internal NiCd battery pack, intended to
power the receiver if the AC power fails. It's unusual
to find a used monitor receiver with the NiCd pack
still capable of holding a charge. More often, the
batteries have died and the series charging resistor
has overheated and burned. Be sure to remove the
dead batteries as they often leak.
If you find the squelch on your Plectron 700 series
receiver has too much hysteresis, replace R96, a
180K resistor, with a 560K resistor. On the P1, the
resistor is designated R81.
Minimum Volume Setting
Both Plectron and Motorola Alert monitor receivers
are designed purposely to have a minimum volume
setting which is still audible. The intent was to
prevent firemen from turning the volume down
completely, forgetting it was down, then missing an
important call.
This "feature" was usually implemented by having a
fixed value resistor in series between one end of the
volume control and ground. In the R15X5, R14X5,
R15X5, and R16X5, a 47 ohm resistor, designated
R624, was inserted in series with the violet wire
leading from the volume control.
You could decrease the lowest volume setting by
decreasing the value of this fixed resistor.
Alternatively, you could replace the resistor with a
jumper wire to completely silence the receiver at
minimum setting of the volume control.
Tone Alert Feature
I have not used the Plectron tone alerting features
so I cannot address that except to say that the
special coils and capacitors for the dual tone
decoders vary depending on the tone frequencies
and are somewhat difficult to obtain.
Repair and Replacement Parts
Repair service and replacement parts for the
Plectron P1 and R700 models is offered by:
Weber Electronics
PO Box 212
5138 Laurel Ln.
Broad Run, VA 20137
telephone (540) 347-7760
You can also obtain Plectron repairs and parts from:
Northeast Communications
The Urban DX’
er
4 Oak Ridge Rd
Newfoundland NJ 07435
201-697-8804
Alert Monitor parts are available from Motorola.
Phone (800)972-3226.
Plectron SM-series Scanning Receivers
Plectron also sold an SM series of scanners,
manufactured in Japan by the same folks who made
Craig (division of Pioneer) scanners. The SM series
are consumer-grade models and are included in this
article for the sake of completeness. They are
crystal controlled radios and feature priority scan
and a separate trimmer capacitor for each channel
to net the crystals on frequency.
Band
Frequency Limits
___________________________
VHF-low 30 - 50
VHF-high 150 - 174
UHF
450 - 470 MHz
Table 4. Plectron SM Model Band Limits
Model UHF VHF-high VHF-low
_________________________________
SM-301 X
X
X
SM-302
X
X
SM-303 X
X
SM-304 X
X
Table 5. Plectron SM Models
The crystals used in the SM series are quite
different from the crystals used in the conventional
single channel Plectron monitors. Here are the
crystal formulas:
VHF low band:
crystal 3rd overtone freq = receive freq + 10.7
MHz
VHF high band:
crystal 3rd overtone freq = (receive freq - 10.7
MHz)/3
UHF band:
crystal 3rd overtone freq = (receive freq - 10.7
MHz) /
10